In existing air-conditioning apparatuses, a method is dominant in which a motor used in a compressor, a fan, or the like is driven using an inverter circuit in order to operate the motor with high efficiency so as to achieve energy savings. In this method, alternating-current power supplied from a commercial power supply is temporarily converted into direct-current power by an AC-DC converter, and the converted direct-current power is converted into alternating-current power having a certain voltage and frequency by an inverter and supplied to the motor. For further energy savings, in recent years, a power conversion apparatus has been known in which a DC-DC converter (booster circuit) is provided on the input side of an inverter. This booster circuit controls an input current in accordance with an ON/OFF state of a semiconductor switching element. Then, there is known a method in which a rectified output of an AC-DC converter is raised in voltage in the booster circuit and is input to the inverter, or a method in which an alternating-current voltage is converted into a direct-current voltage by using a PWM converter.
Incidentally, when a load connected to a power system changes suddenly, or when the power system is, for example, struck by lightning, a brief system shutdown called a momentary power failure may occur. Furthermore, a voltage sag, which is a momentary reduction in voltage, may occur due to a lightning strike or the like. Since the booster circuit controls an input current in accordance with ON/OFF operation of the semiconductor switching element, if a sudden change in power-supply voltage due to a momentary power failure or a momentary voltage sag occurs, an overcurrent occurs in the semiconductor switching element, and an output voltage from a converter unit may change suddenly. At this time, operation of the inverter located at a stage subsequent to the converter unit becomes unstable, and operation of a load, such as a motor, may stop due to a loss of synchronization.
Thus, hitherto, various techniques have been proposed to prevent a malfunction due to a momentary power failure or a momentary voltage sag (see, for example, Patent Literatures 1 and 2). Patent Literature 1 discloses a stabilized power supply apparatus that includes an abnormality detection circuit to detect an abnormality in input voltage, and that effects emergency stopping of a converter unit when the abnormality detection circuit detects an abnormality. Patent Literature 2 discloses a power conversion apparatus that determines, on the basis of a variation in current value, the occurrence of an abnormality in which a power-supply voltage temporarily increases.